tools lib bpf: Don't do a feature check when cleaning

Before this patch libbpf always do feature check even when cleaning.

For example:

  $ cd kernel/tools/lib/bpf
  $ make

  Auto-detecting system features:
  ...                        libelf: [ on  ]
  ...                           bpf: [ on  ]

    CC       libbpf.o
    CC       bpf.o
    LD       libbpf-in.o
    LINK     libbpf.a
    LINK     libbpf.so
  $ make clean
    CLEAN    libbpf
    CLEAN    core-gen
  $ make clean

  Auto-detecting system features:
  ...                        libelf: [ on  ]
  ...                           bpf: [ on  ]

    CLEAN    libbpf
    CLEAN    core-gen
  $

Although the first 'make clean' doesn't show feature check result, it
still does the check. No output because check result is similar to
FEATURE-DUMP.libbpf.

This patch uses same method as perf to turn off feature checking when
'make clean'.

Reported-and-Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1448372181-151723-3-git-send-email-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/bpf/Makefile

@@ -71,7 +71,17 @@ FEATURE_DISPLAY = libelf bpf
 INCLUDES = -I. -I$(srctree)/tools/include -I$(srctree)/arch/$(ARCH)/include/uapi -I$(srctree)/include/uapi
 FEATURE_CHECK_CFLAGS-bpf = $(INCLUDES)
 
+check_feat := 1
+NON_CHECK_FEAT_TARGETS := clean TAGS tags cscope help
+ifdef MAKECMDGOALS
+ifeq ($(filter-out $(NON_CHECK_FEAT_TARGETS),$(MAKECMDGOALS)),)
+  check_feat := 0
+endif
+endif
+
+ifeq ($(check_feat),1)
 include $(srctree)/tools/build/Makefile.feature
+endif
 
 export prefix libdir src obj